1. Field of the Invention
The present invention relates to an inkjet printer and a paper feeding method, and, more particularly, to an inkjet printer which picks up papers one by one from a cassette and moves the paper to a printing unit that prints an image on the paper, and a paper feeding method therefor.
2. Description of the Related Art
Generally, an inkjet printer picks up a single paper from a cassette containing a stack of paper and moves the paper. A single paper is separated from a stack of paper and picked up using a friction or a stiffness of the paper.
FIG. 1 illustrates a method of picking up a single paper using a stiffness of the paper. Referring to FIG. 1, a cassette 51 contains a paper stack P. A pickup roller 52 is placed on the paper stack P to be in close contact with and press the paper stack P. A resistance element 53 is obliquely placed in front of the paper stack P at an inclination angle A with respect to the front of the paper stack P. When the pickup roller 52 rotates, papers are moved. Here, two or more papers are usually moved due to a frictional force between the papers. A frictional force between the pickup roller 52 and the paper stack P is greater than a frictional force between papers in the paper stack P. Accordingly, when papers contact the resistance element 53, only a paper P1 that is in contact with the pickup roller 52 on a top of the paper stack P slips over a paper therebelow and is curved at its front portion meeting the resistance element 53, thereby having a curl, as denoted by a reference character C1. Thereafter, with rotation of the pickup roller 52, the curl becomes greater as denoted by a reference character C2, and the curl is suddenly straightened as denoted by a reference character C3 due to a stiffness of the paper P1, so that only the paper P1 is separated from the paper stack P and fed.
The inclination angle A of the resistance element 53 is set to be suitable to widely used plain papers. Accordingly, when a thick paper having a large stiffness is used, a separation procedure denoted by C1, C2, and C3 may be never performed or may be incompletely performed after a front end of the thick paper reaches the resistance element 53. As a result, a pickup error, in which more than one paper or no paper is fed, occurs. In addition, a driving motor (not shown) to drive the pickup roller 52 may be overloaded.
To overcome this problem, increasing the inclination angle A of the resistance element 53 is necessary. Referring to FIG. 2, the resistance element 53 is rotatably installed and is elastically biased by a spring 54. When a thick paper P1 having a large stiffness is moved, the resistance element 53 rotates in a direction B so that a load on the driving motor (not shown) is decreased and the separation procedure denoted by C1, C2, C3 is performed. However, such a structure allows the inclination angle A to be slightly changed even for the plain paper, which may cause double feeding.
FIG. 3 schematically illustrates a paper feed path of a conventional inkjet printer. Referring to FIG. 3, a paper P is picked up from a cassette 51 by a pickup roller 52 and is inserted into a feed roller 63 via a drive roller 62. The feed roller 63 feeds the paper P according to a print speed of a printing unit 64. Reference numeral 66 denotes a first paper detector which detects a front end of the paper P. When the paper P is not detected by the first paper detector 66 within a predetermined time, a paper jam is determined to have occurred between the cassette 51 and the first paper detector 66. Reference numeral 67 denotes a second paper detector which detects the front end of the paper P fed by the feed roller 63. The second paper detector 67 is provided to adjust a print start time of the printing unit 64. When the paper P is not detected by the second paper detector 67 within a predetermined time since detection of the paper P by the first paper detector 66, a paper jam is determined to have occurred between the first and second paper detectors 66 and 67.
To feed the paper P without a skew, registration is performed between the first paper detector 66 and the feed roller 63. After the front end of the paper P is detected by the first paper detector 66, the paper P is moved slightly further than a distance between the first paper detector 66 and the feed roller 63. Here, the feed roller 63 rotates in a direction opposite to a paper feed direction or does not rotate. Then, due to a stiffness of the paper P, a front portion of the paper P is curved as expressed by a dotted line, and the front end of the paper P is aligned with the feed roller 63.
As described above, a mis-feed of the paper P occurring between the first and second paper detectors 66 and 67 can be determined based on whether the paper P is detected by the second paper detector 67 within the predetermined time since the paper P was detected by the first paper detector 66. During registration, when the paper P is fed only to a position X shown in FIG. 3 due to a mis-feed, the front end of the paper P is not detected by the second paper detector 67 within the predetermined time, and thus, a paper jam is determined. However, even when the paper P is not in state expressed by the dotted curve in FIG. 3, if only the front end of the paper P becomes in contact with the feed roller 63, the second paper detector 67 detects the paper P within the predetermined time. As a result, the paper jam is not determined, and printing unit 64 prints an image onto the paper. In this situation, since the front end of the paper P is not exactly aligned with the feed roller 63, that is, since a registration error occurs, a skew of the paper may not be detected.